`First Light' in the Universe; What Ended the "Dark Age"?

The universe would have been completely dark between the epoch of recombination and the development of the first non-linear structure. But at redshifts beyond 5 -- perhaps even beyond 20 -- stars formed within `subgalaxies' and created the first heavy elements; these same systems (together perhaps with `miniquasars') generated the UV radiation that ionized the IGM, and maybe also the first significant magnetic fields. Although we can already probe back to $z \simeq 5$, these very first objects may be so faint that their detection must await next-generation optical and infrared telescopes. Observations in other wavebands may offer indirect clues to when reionization occurred. Despite the rapid improvements in numerical simulations, the processes of star formation and feedback are likely to remain a challenge for the next decade.Comment: For ``Physics Reports'' special issue in memory of D.N. Schram

Soft X-ray Absorption by High-Redshift Intergalactic Helium

The Lyman alpha absorption from intergalactic, once-ionized helium (HeII) has been measured with HST in four quasars over the last few years, over the redshift range 2.4 < z < 3.2. These observations have indicated that the HeII reionization may not have been completed until z\simeq 2.8, and that large fluctuations in the intensity of the HeII-ionizing background were present before this epoch. The detailed history of HeII reionization at higher redshifts is, however, model-dependent and difficult to determine from these observations, because the IGM can be completely optically thick to Lya photons when only a small fraction of the helium remains as HeII. In addition, finding quasars in which the HeII Lya absorption can be observed becomes increasingly difficult at higher redshift, owing to the large abundance of hydrogen Lyman limit systems. It is pointed out here that HeII in the IGM should also cause detectable continuum absorption in the soft X-rays. The spectrum of a high-redshift source seen behind the IGM when most of the helium was HeII should recover from the HeII Lyman continuum absorption at an observed energy \sim 0.1 keV. Galactic absorption will generally be stronger, but not by a large factor; the intergalactic HeII absorption can be detected as an excess over the expected Galactic absorption from the 21cm HI column density. In principle, this method allows a direct determination of the fraction of helium that was singly ionized as a function of redshift, if the measurement is done on a large sample of high-redshift sources over a range of redshift.Comment: accepted to The Astrophysical Journal Letter

Signatures of the Origin of High-Energy Cosmic Rays in Cosmological Gamma-Ray Bursts

We derive observational consequences of the hypothesis that cosmic rays (CR's) of energy $>10^{19}eV$ originate in the same cosmological objects producing gamma-ray bursts (GRB's). Inter-galactic magnetic fields $\gtrsim 10^{-12} G$ are required in this model to allow CR's to be observed continuously in time by producing energy dependent delays in the CR arrival times. This results in individual CR sources having very narrow observed spectra, since at any given time only those CR's having a fixed time delay are observed. Thus, the brightest CR sources should be different at different energies. The average number of sources contributing to the total CR flux decreases with energy much more rapidly than in a model of steady CR sources, dropping to one at $E_{crit} \simeq2\times10^{20}$~eV with very weak sensitivity to the inter-galactic magnetic field strength. Below $E_{crit}$, a very large number of sources is expected, consistent with observations. Above $E_{crit}$, a source may be observed with a flux considerably higher than the time-averaged CR flux from all sources, if a nearby GRB occurred recently. If such a source is present, its narrow spectrum may produce a ``gap'' in the total spectrum. These signatures should be detectable by the planned ``Auger'' CR experiment.Comment: Submitted to the ApJ Letters; 10 pages (LaTeX file, 3 PostScript figures, aaspp4.sty style file, tar'ed, gzip'ed and uuencoded

Cosmological implications of baryon acoustic oscillation measurements

We derive constraints on cosmological parameters and tests of dark energy models from the combination of baryon acoustic oscillation (BAO) measurements with cosmic microwave background (CMB) data and a recent reanalysis of Type Ia supernova (SN) data. In particular, we take advantage of high-precision BAO measurements from galaxy clustering and the Lyman-α forest (LyaF) in the SDSS-III Baryon Oscillation Spectroscopic Survey (BOSS). Treating the BAO scale as an uncalibrated standard ruler, BAO data alone yield a high confidence detection of dark energy; in combination with the CMB angular acoustic scale they further imply a nearly flat universe. Adding the CMB-calibrated physical scale of the sound horizon, the combination of BAO and SN data into an 'inverse distance ladder' yields a measurement of H0=67.3 ±1.1 km s-1 Mpc-1 , with 1.7% precision. This measurement assumes standard prerecombination physics but is insensitive to assumptions about dark energy or space curvature, so agreement with CMB-based estimates that assume a flat Λ CDM cosmology is an important corroboration of this minimal cosmological model. For constant dark energy (Λ ), our BAO +SN +CMB combination yields matter density Ωm=0.301 ±0.008 and curvature Ωk=-0.003 ±0.003 . When we allow more general forms of evolving dark energy, the BAO +SN +CMB parameter constraints are always consistent with flat Λ CDM values at ≈1 σ . While the overall χ2 of model fits is satisfactory, the LyaF BAO measurements are in moderate (2 - 2.5 σ ) tension with model predictions. Models with early dark energy that tracks the dominant energy component at high redshift remain consistent with our expansion history constraints, and they yield a higher H0 and lower matter clustering amplitude, improving agreement with some low redshift observations. Expansion history alone yields an upper limit on the summed mass of neutrino species, ∑mν<0.56 eV (95% confidence), improving to ∑mν<0.25 eV if we include the lensing signal in the Planck CMB power spectrum. In a flat Λ CDM model that allows extra relativistic species, our data combination yields Neff=3.43 ±0.26 ; while the LyaF BAO data prefer higher Neff when excluding galaxy BAO, the galaxy BAO alone favor Neff≈3 . When structure growth is extrapolated forward from the CMB to low redshift, standard dark energy models constrained by our data predict a level of matter clustering that is high compared to most, but not all, observational estimates

The 21cm Signature of the First Stars

We predict the 21-cm signature of the first metal-free stars. The soft X-rays emitted by these stars penetrate the atomic medium around their host halos, generating Lyman alpha photons that couple the spin and kinetic temperatures. These creates a region we call the Lyman alpha sphere, visible in 21-cm against the CMB, which is much larger than the HII region produced by the same star. The spin and kinetic temperatures are strongly coupled before the X-rays can substantially heat the medium, implying that a strong 21-cm absorption signal from the adiabatically cooled gas in Hubble expansion around the star is expected when the medium has not been heated previously. A central region of emission from the gas heated by the soft X-rays is also present although with a weaker signal than the absorption. The Lyman alpha sphere is a universal signature that should be observed around any first star illuminating its vicinity for the first time. The 21-cm radial profile of the Lyman alpha sphere can be calculated as a function of the luminosity, spectrum and age of the star. For a star of a few hundred solar masses and zero metallicity (as expected for the first stars), the physical radius of the Lyman alpha sphere can reach tens of kiloparsecs. The first metal-free stars should be strongly clustered because of high cosmic biasing; this implies that the regions producing a 21-cm absorption signal may contain more than one star and will generally be irregular and not spherical, because of the complex distribution of the gas. We discuss the feasiblity of detecting these Lyman alpha spheres, which would be present at redshifts $z\sim 30$ in the Cold Dark Matter model. Their observation would represent a direct proof of the detection of a first star.Comment: replaced with ApJ accepted version. Many minor revisions and additional references, major results unchange

Damped Lyman-alpha and Lyman Limit Absorbers in the Cold Dark Matter Model

We study the formation of damped \lya and Lyman limit absorbers in a hierarchical clustering scenario using a gas dynamical simulation of an $\Omega = 1$, cold dark matter universe. In the simulation, these high column density systems are associated with forming galaxies. Damped \lya absorption, N_{HI} \simgt 10^{20.2}\cm^{-2}, arises along lines of sight that pass near the centers of relatively massive, dense protogalaxies. Lyman limit absorption, 10^{17}\cm^{-2} \simlt N_{HI} \simlt 10^{20.2}\cm^{-2}, develops on lines of sight that pass through the outer parts of such objects or near the centers of smaller protogalaxies. The number of Lyman limit systems is less than observed, while the number of damped \lya systems is quite close to the observed abundance. Damped absorbers are typically $\sim 10$ kpc in radius, but the population has a large total cross section because the systems are much more numerous than present day $L_*$ galaxies. Our results demonstrate that high column density systems like those observed arise naturally in a hierarchical theory of galaxy formation and that it is now possible to study these absorbers directly from numerical simulations.Comment: compressed postscript, 12 pages including 2 embedded figures. A version that also includes embedded Figure 1, a 6 Mbyte color postscript image (which prints reasonable grey scale on a b/w printer) is available from ftp://bessel.mps.ohio-state.edu/pub/dhw/Preprints Submitted to ApJ Letter